ORGAN OF SIGHT. 113 



over the eye, and becomes transparent where it enters the 

 orbit ; sometimes it simply passes over the orbit, sometimes 

 it forms a circular fold. The anterior and posterior portions 

 may be especially broad and the seat of an adipose deposit 

 {adipose eyelids), as in Scomber, GaraTix, Mugil, etc. In many 

 of these fishes the extent of these eyelids varies with the 

 seasons; during the spawning season they are so much loaded 

 with fat as nearly to hide the whole eye. Many Sharks pos- 

 sess a nictitating membrane, developed from the lower part of 

 the palpebral fold, and moved by a proper set of muscles. 



The form of the hulhus (Fig. 48) is subhemispherical, the 

 cornea (co) being flat. If it were convex, as ia higher Verte- 

 brates, it would be more liable to injury ; but being level with 

 the side of the head the chances of injury by friction are dim- 

 inished. The sclerotica (sc) is cartilaginous in Chondropterygians 

 and Acipensers, fibrous and of varying thickness in Teleosteans, 

 in the majority of which it is supported 

 by a pair of cartilaginous or ossified 

 hemispheroid cups (c). In a few fishes, 

 as in Ceratodus, Xiphias, the cups are 

 confluent into one cup, which possesses 

 a foramen behind to allow the passage 

 of the optic nerve (o). The cornea of ^jg. 43. 



Anahleps shows an unique peculiarity. Vertical section througii 



J . -, -, T 1 1 ■ i. 1 eye of XipMas. (After 



It IS crossed by a dark horizontal ^^^^-^ 



stripe of the conjunctiva, dividing it m, Comea ; ac, sclerotica ; 

 . , in i- 1 ii, d nervns opticus ; e, 



into an upper and lower portion; also the ^^^^^^.^ ^^^^^j^. ^_ 



iris is perforated by two pupils. This fish membrana argentea ; 



iB observed to swim frequently with half I^rJ^^ra^t:; 

 of its head out of the water, and it is a choroid gland; r, re- 

 fact that it can see out of the water as t!°^ - /■ P™f «7 ^^i- 



ciformis ; h, humor 

 well as in it. vitreus ; I, lens ; i. 



The membranes situated between J™- 

 the sclerotica and retina are collectively called choroidea, and 



I 



